An optical fiber for laser beam guiding is used in a catheter for the use of lasing which removes a lesion portion such as a thrombus in a blood vessel, which a practitioner cannot directly approach, by means of irradiating with a laser beam and vaporizing the portion. In such the lasing catheter, the optical fiber is used to transmit the laser beam to a tip of the catheter with a low beam loss.
In the meantime, in the lasing, it is required for the cure to use a beam having a short wavelength which has a large amount of energy and is sufficiently absorbed in a tissue in vivo.
However, a threshold value of destruction of the optical fiber for laser beam guiding is sharply reduced as the wavelength of the laser beam injected therein becomes short. For example, in the case of quartz, it is said that while the threshold value of destruction for the fundamental wave (1.064 .mu.m) of YAG laser beam is 130 GW/cm.sup.2, the triple-frequency wave (0.355 .mu.m) is reduced to one fifth of that for the fundamental wave.
Thus, even though lasing with a beam having the short wavelength such as an ultraviolet light (UV light) is effective for the cure, it has not been sufficiently utilized since the threshold value of destruction for the laser beam having the short wavelength is low.